Systems and methods for guiding the drilling of a horizontal well

ABSTRACT

System and methods for guiding the drilling of a horizontal well are disclosed. A current is provided in at least one conductor positioned in a target vertical well. A magnetic field generated by the current is measured at a drilling assembly that is drilling the horizontal well. A direction from the drilling assembly to the target vertical well is determined based at least in part on the measured magnetic field. A distance from the drilling assembly to the target vertical well is determined based at least in part on the measured magnetic field. The determination of the distance includes determining at least one gradient.

FIELD OF THE INVENTION

The invention relates generally to systems and methods for guiding thedrilling of a horizontal well and, more specifically, to systems andmethods for guiding the drilling of a horizontal well into anintersection with a vertical well.

BACKGROUND OF THE INVENTION

Directional drilling, or slant drilling, is typically utilized to drillnon-vertical wells. Directional drilling is utilized in a wide varietyof applications, including oil drilling, utility installation drilling,and in-seam drilling. In some applications, directional drilling isutilized to intersect an existing vertical well with a horizontal well.Various techniques and methods are typically utilized in an attempt toaccurately intersect the vertical well with the horizontal well that isbeing drilled.

One conventional technique that is utilized is to rely on directionalsurveys in an attempt to place the horizontal well in close proximity toa vertical target well. However, this technique may often be ineffectivedue to errors in the surveying of the two wells and the land survey atthe surface.

Other conventional techniques utilize various sensors that are intendedto assist in the guiding of the intersection of the horizontal well witha vertical well. The sensors may be situated either in the drillingassembly of the horizontal well with a signal source in the existingvertical well or, alternatively, in the existing vertical well with asignal source in the drilling assembly of the horizontal well. However,many of these conventional techniques may also include small butinherent errors in determining the intersection of the vertical andhorizontal wells. Errors in estimating the positions of both thevertical and horizontal wells can accumulate as drilling and surveyingprogresses. Eventually, the additive errors impacting the path ortrajectory may be so large as to prevent a desired near intersection ofthe two wells.

Additionally, it can be difficult to accurately determine a range ordistance between a horizontal well that is being drilled and a targetvertical well utilizing the prior art techniques. A failure toaccurately determine the range or distance often leads to failedintersections or poor intersections between the two wells. A poordetermination of range or distance may prevent a driller that is guidingthe drilling of the horizontal well from being able to plan and takecorrective maneuvers in steering the drilling of the horizontal well.Thus, without an accurate determination of range or distance, a drillermay not be able to correct the direction of the drilling of thehorizontal well in order to intersect a vertical well.

Accordingly, there is a need for improved systems and methods forguiding the drilling of a horizontal well in order to intersect avertical well.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a method is provided forguiding the drilling of a horizontal well. A current is provided in atleast one conductor positioned in a target vertical well. A magneticfield generated by the current is measured at a drilling assembly thatis drilling the horizontal well. A direction from the drilling assemblytowards the target vertical well is determined based at least in part onthe measured magnetic field. A distance from the drilling assembly tothe target vertical well is determined based at least in part on themeasured magnetic field. The determination of the distance also includesat least one gradient.

According to another aspect of the invention, a system is provided forguiding the drilling of a horizontal well. The system can include atleast one conductor, one or more sensors, and a control unit. The atleast one conductor is positioned in a target vertical well. The atleast one conductor carries a current signal. The one or more sensorsare associated with a drilling assembly that is drilling the horizontalwell. The one or more sensors measure the intensity of a magnetic fieldgenerated by the current signal. The control unit receives the intensitymeasurements from the one or more sensors. The control unit determines adirection from the drilling assembly towards the target vertical wellbased at least in part on the received intensity measurements. Thecontrol unit determins a distance from the drilling assembly to thetarget vertical well based at least in part on the received intensitymeasurements. At least one gradient calculation is utilized to determinethe distance from the drilling assembly to the target vertical well.

Other aspects of the invention will become apparent from the followingdescription taken in conjunction with the following drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a schematic diagram of an exemplary intersection of a verticalwell by a horizontal well in accordance with an illustrative aspect ofthe invention.

FIG. 2 is a schematic diagram of a target vertical well, according to anillustrative aspect of the invention.

FIG. 3 is a schematic diagram of an exemplary drilling assembly that isutilized to detect a target vertical well, according to an illustrativeaspect of the invention.

FIG. 4 is a schematic diagram that depicts the current direction of thedrilling assembly and the target direction to the target vertical well,according to an illustrative aspect of the invention.

FIG. 5A is a top view schematic diagram of the drilling of a horizontalwell to form an intersection or near-intersection with a vertical well,according to an illustrative aspect of the invention.

FIG. 5B is a cross sectional view taken along the line A to A′ of thedrilling of a horizontal well to form an intersection ornear-intersection with a vertical well, according to an illustrativeaspect of the invention.

FIG. 6 is a top view schematic diagram of exemplary adjustments that aremade during the drilling of a horizontal well to form an intersection ornear-intersection with a target vertical well, according to anillustrative aspect of the invention.

FIG. 7 is a block diagram of an exemplary control unit that is utilizedin accordance with certain aspects of the invention.

FIG. 8 is an exemplary flowchart of the general operation of the controlunit of FIG. 7, according to an illustrative aspect of the invention.

FIG. 9 is a graphical representation of one example of falloff rates fortwo different alternating current magnetic fields having differentintensities, in accordance with an illustrative aspect of the invention.

FIG. 10A is an exemplary flowchart depicting the operations that can betaken by the control unit of FIG. 7 to determine a distance or range toa target vertical well, in accordance with an illustrative aspect of theinvention.

FIG. 10B is a graphical representation of the distances and values thatcan be measured and/or determined by utilizing the exemplary operationsdepicted in FIG. 10A, in accordance with an illustrative aspect of theinvention.

FIG. 11 is an exemplary flowchart depicting the operations that can betaken by the control unit of FIG. 7 to determine a direction to a targetvertical well, in accordance with an illustrative aspect of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Illustrative aspects of the inventions now will be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all aspects of the inventions are shown. Indeed, the inventionmay be embodied in many different forms and should not be construed aslimited to the aspects set forth herein; rather, these aspects areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout thedescription and drawings.

The invention is described below with reference to block diagrams ofsystems, methods, apparatuses and computer program products according toan aspect of the invention. It will be understood that each block of theblock diagrams, and combinations of blocks in the block diagrams,respectively, can be implemented by computer program instructions. Thesecomputer program instructions may be loaded onto a general purposecomputer, special purpose computer, or other programmable dataprocessing apparatus to produce a machine, such that the instructionswhich execute on the computer or other programmable data processingapparatus create means for implementing the functionality of each blockof the block diagrams, or combinations of blocks in the block diagramsdiscussed in detail in the descriptions below.

These computer program instructions may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instructions thatimplement the function specified in the block or blocks. The computerprogram instructions may also be loaded onto a computer or otherprogrammable data processing apparatus to cause a series of operationalsteps to be performed on the computer or other programmable apparatus toproduce a computer implemented process such that the instructions thatexecute on the computer or other programmable apparatus provide stepsfor implementing the functions specified in the block or blocks.

Accordingly, blocks of the block diagrams support combinations of waysto perform the specified functions, combinations of steps for performingthe specified functions and program instruction for performing thespecified functions. It will also be understood that each block of theblock diagrams, and combinations of blocks in the block diagrams, can beimplemented by special purpose hardware-based computer systems thatperform the specified functions or steps, or combinations of specialpurpose hardware and computer instructions.

The invention may be implemented through an application program runningon an operating system of a computer. The invention also may bepracticed with other computer system configurations, including hand-helddevices, multiprocessor systems, microprocessor based or programmableconsumer electronics, mini-computers, mainframe computers, etc.

Application programs that are components of the invention may includeroutines, programs, components, data structures, etc. that implementcertain abstract data types, perform certain tasks, actions, or tasks.In a distributed computing environment, the application program (inwhole or in part) may be located in local memory, or in other storage.In addition, or in the alternative, the application program (in whole orin part) may be located in remote memory or in storage to allow for thepractice of the inventions where tasks are performed by remoteprocessing devices linked through a communications network. Exemplaryaspects of the invention will be described with reference to thefigures, in which like numerals indicate like elements throughout theseveral drawings.

Disclosed are aspects of systems and methods for guiding or controllingthe drilling of a horizontal well or hole so as to intersect or verynearly intersect an existing vertical well or a series of verticalwells. As used herein, the term “horizontal” means parallel to orapproximately parallel to level ground or the plane of the horizon. Asused herein, the term “vertical” means being in a position or directionthat is perpendicular to or approximately perpendicular to the plane ofthe horizon. The vertical wells to be intersected may be disposed moreor less in a straight line; however, it will be appreciated that thehorizontal well may be utilized to intersect other vertical wellarrangements.

A signal may be transmitted, injected, or otherwise communicated into anexisting vertical well that is a target for the intersection or nearintersection by a horizontal well that is to intersect the verticalwell. The drilling assembly utilized in the drilling of the horizontalwell may include or contain sensing components that are capable ofidentifying the signal in the vertical well. Once the signal in thevertical well has been identified, the location of the vertical well inrelation to the drilling assembly may be accurately determined.

Both a direction from the drilling assembly to the vertical well and arange or distance between the drilling assembly and vertical well may bedetermined. Based at least in part on these determinations, the steeredpath, course, or trajectory of the horizontal well may he controlled soas to bring about an intersection or near-intersection of the horizontalwell and the target vertical well. Other target vertical wells may thenbe located and intersected by the horizontal well by utilizing the sametechnique. Accordingly, a single horizontal well may intersect a secondwell or multiple vertical wells.

FIG. 1 is a schematic diagram of an exemplary application 100 in which avertical well 105 is intersected by a horizontal well 110 in accordancewith an illustrative aspect of the invention. It will be appreciatedthat the systems, methods, and techniques described in aspects of theinvention may be utilized in a wide variety of applications. Forexample, aspects of the invention may be utilized in the drilling ofhorizontal wells in conjunction with collecting hydrocarbon products.

Certain hydrocarbon products such as gases, for example, methane oftenfound captured in relatively shallow coal deposits (also referred to ascoal bed methane, or CBM) are usually held in place by the hydrostaticpressure of water. In order to collect these gases or other hydrocarbonproducts, the partial pressure of the water can be reduced by a suitable“dewatering” process. The “dewatering” process removes a critical amountof water in order to release the gases and induce a flow of the gasestoward the wellbore of a nearby well, such as, for example, the verticalwell 105 of FIG. 1. The gases or other hydrocarbon products are thenextracted from the well, compressed, and piped to market. In order tocomplete the “dewatering,” a vertical well 105 can be underreamed withina coal seam or coal seams.

The underreaming functions to enlarge a section 115 of the wellborewhich is within a coal seam past its original drilled size. The wellborecan be enlarged to any size such as, for example, to a diameter ofseveral feet. The enlarged wellbore section 115 acts as a reservoir orsump for collection of the water, thereby allowing a more efficientremoval of water in order to collect the gases. A horizontal well 110that is being drilled is steered into an intersection ornear-intersection with the vertical well 110 near the enlarged section115 of the wellbore in order to remove the water and later to producethe methane gas flowing into the enlarged section 115.

An extension of this method utilizes a steered horizontal well 110 tointerconnect multiple vertical wells, thereby further increasingefficiency in collecting gases. A further extension of the method is todrill a complex pattern of horizontal wells and interconnected passagesbetween strategically placed vertical wells. Such a configuration ofhorizontal and vertical wells reduces the number of surface locationsneeded to facilitate drilling and collection of gases. Accordingly, sucha configuration may be more efficient and may reduce the environmentalimpact caused at the surface.

According to aspects of the invention, at least one vertical well 105 isintersected or nearly intersected by a horizontal well 110. The drillingof the horizontal well 110 can be precisely or accurately guidedutilizing a measurement while drilling (MWD) system, technique, and/ormethod. Although aspects of the invention are described herein asutilizing a measurement while drilling (MWD) system or technique, itwill be appreciated that other tools, systems, techniques and/or methodscan be utilized in accordance with aspects of the invention. Adirectional driller (not shown) that is directing or guiding thedrilling of the horizontal well 110 is supplied with data or informationthat will assist in completing the intersection or near-intersection.The directional driller can be an individual, entity and/or a system orsystem component that is guiding the drilling of the horizontal well110. The directional driller can be located at or associated with asurface location 117 at which the drilling of the horizontal well 110 iscommenced. The directional driller guides a steerable drilling assembly120 that is utilized to drill the horizontal well. The data provided tothe directional driller can include information associated with adirection from the drilling assembly 120 to the target vertical well 105and information associated with a distance from the drilling assembly tothe target vertical well 105. The data allows the directional driller toorient the drilling assembly 120 and control the placement and drillingof the horizontal well 110. Additionally, the data allows an accurateintersection or near-intersection between the horizontal well 110 andthe enlarged section 115 of the vertical well 105. The accurateintersection or near-intersection achieved by utilizing the data allowsa reduction of the size needed for the enlarged section 115 of thevertical well 105. By increasing the accuracy in guiding the drillingassembly 120 to the target vertical well 105, both the size of theenlarged section 115 and the time needed to underream and clean thevertical well 105 can be reduced.

With continued reference to FIG. 1, the drilling assembly 120 includesat least one motor 125 and at least one drill bit 130. The drillingassembly 120 also includes one or more sensors 135 that are utilized tolocate the target vertical well 105. A wide variety of techniques,methods, or systems can be utilized to locate the target vertical well105. According to an aspect of the invention, the one or more sensors135 are sensors that are capable of detecting a magnetic field such as,for example, a magnetic field that is generated or created by analternating current. One or more conductors 140 are positioned in thevertical well 105 or near the vertical well 105. The one or moreconductors 140 are utilized to carry a current through at least aportion of the length of the vertical well 105. The current is suppliedto the one or more conductors 140 by an appropriate current generationsource such as, for example, a suitable alternating current generationsource 145.

The alternating current generation source 145 is connected to one end ofthe one or more conductors 140. An alternating current is driven throughthe one or more conductors 140 in order to generate alternating currentmagnetic fields that can be detected by the one or more sensors 135 inthe drilling assembly 120. The one or more conductors 140 are alsoconnected to or terminated at a ground 150 such as, for example, toearth ground. The one or more conductors 140 are terminated at a ground150, such as earth ground, at their distal ends. By connecting the oneor more conductors 140 to a ground 150, the one or more conductors 140create a discrete source for detection by the one or more sensors 135,as explained in greater detail below with reference to FIG. 2.

Additionally, at least a portion of the vertical well 105 may include anappropriate casing 155, as will be understood by those skilled in theart. The casing 155 can be any suitable well casing or combination ofwell casings. Similarly, at least a portion of the horizontal well 110may include an appropriate casing 160. The casing 160 can be anysuitable well casing or combination of well casings.

FIG. 2 is a schematic diagram of a target vertical well 105, accordingto an illustrative aspect of the invention. One or more conductors,similar to 140 in FIG. 1, are lowered into the target vertical well 105.The one or more conductors, such as 140, make up a wireline 205. Thewireline 205 is lowered into the target vertical well 105 by a suitablewireline lowering device 207. The wireline 205 is extended from thesurface through the target vertical well 105, and the wireline 205 isterminated to a ground, similar to 150 in FIG. 1, at its distal end. Thewireline 205 includes a grounding electrode 210 that forms a suitableconnection to earth ground.

The grounding electrode 210 is any suitable contact electrode that formsa connection to earth ground, as will be understood by those of skill inthe art. At the surface, one side of an alternating current generationsource, shown as 145 and similar to that shown in FIG. 1, is connectedto the wireline 205, and the alternating current generation source, suchas 145, drives an alternating current into the wireline 205. Theopposite side of the alternating current generation source, such as 145,is connected to a ground, such as 150, for example, to earth ground.Accordingly, an alternating current is driven through the wireline 205and allowed to return to the alternating current generation source, suchas 145, through the earth. The alternating current that is communicatedonto or present in the wireline 205 creates or generates one or morealternating current magnetic fields or lines of magnetic flux, such as215. Given an elongated wireline 205, the alternating current magneticfields propagate in perpendicular or substantially perpendicular mannerfrom the wireline 205. At the end of the wireline 205, the groundingelectrode 210 functions to diffuse the alternating current into theearth. The alternating current magnetic field may have many differentintensities and/or frequencies. For example, the alternating currentmagnetic field may be a low-frequency field.

In the horizontal well, shown as 110 in FIG. 2, in progress, thedrilling assembly, such as 120, includes one or more sensors, such as135, that detect the alternating current magnetic fields that areproduced as a result of the current in the wireline 205. The one or moresensors, such as 135, can include any number of sensors that are capableof detecting the alternating current magnetic fields such as, forexample, an orthogonal triad of alternating current sensors. Accordingto an aspect of the invention, the one or more sensors 135 have asensitivity of approximately 0.3 nanoteslas; however, it will beappreciated that sensors with other sensitivities may be utilized asdesired. For example, sensors with a sensitivity of approximately 0.1nanoteslas can be utilized. Additionally, the one or more sensors 135are capable of detecting very low intensities of low-frequencyalternating current magnetic fields. It will be appreciated that themagnitude of the current signal within the wireline 205 can becontrolled so that the alternating current magnetic fields propagatedfrom the wireline 205 have intensities that fall within the sensitivityrange of the one or more sensors 135.

FIG. 3 is a schematic diagram of an exemplary drilling assembly 120 thatis be utilized to detect a target vertical well, such as 105, accordingto an illustrative aspect of the invention. The one or more sensors,such as 135, in the drilling assembly 120 detect the alternating currentmagnetic field that is produced by the wireline 205. The field vector atthe one or more sensors 135 is at a right angle to a target direction tothe target vertical well 105. Additionally, the field vector at the oneor more sensors 135 is at a tangent to the concentric lines of magneticflux that are propagating from the wireline 205.

The alternating current magnetic field is detected and measured by theone or more sensors 135. The one or more sensors 135 then communicatethe measurements to one or more control units that process themeasurements. The information can be communicated via any appropriatecommunication technique(s) or device(s) such as, for example, a mudpulse system, a steering toot probe, a wired connection, a wirelessconnection, a cellular connection, and/or a radio connection. A firstcontrol unit 305 can be included in or associated with the drillingassembly 120. The first control unit 305 can process the measurementsand communicate or transmit to a directional driller informationassociated with a distance and/or a direction from the drilling assembly120 to the target vertical well 105. The communicated information can befurther processed by at least one control unit (not shown) associatedwith the directional driller.

Information associated with the direction and distance to the targetvertical well 105 can be communicated or provided to the directionaldriller in an appropriate form that allows the directional driller toguide the drilling of the horizontal well 110 to achieve an intersectionor near-intersection with the target vertical well 105. With theprovided information, the directional driller can adjust the path of thedrilling assembly 120 as needed to achieve the intersection ornear-intersection within acceptable limits of curvature, or “dogleg” inthe horizontal well 110. Small amounts of dogleg in the horizontal well110 may be acceptable. The acceptable dogleg may vary depending on theapplication, and the acceptable dogleg can be expressed in anyappropriate form such as, for example, in degrees per 100 feet or indegrees per 30 meters. Relatively larger amounts of dogleg or curvatureof the hole may cause problems in the continuation of the drillingand/or in the later installation of pipe in the horizontal well 110.

FIGS. 4-6 are schematic diagrams depicting the guidance of the drillingof a horizontal well 110 to create an intersection or near-intersectionwith a target vertical well 105, according to an illustrative aspect ofthe invention. FIG. 4 is a schematic diagram that depicts the currentdirection 405 of the drilling assembly, shown in FIGS. 1-3 as 120, andthe target direction 420 to the target vertical well 105. As shown inFIG. 4, the target direction 425 to the target vertical well 105 formsapproximately a right angle with a line 410 that is approximatelytangential to the are of the alternating current magnetic field 415 thatis emitted from the wireline 205. An approximate angle θ 425 representsthe difference between the current direction 405 and the targetdirection 420 if such a difference exists.

FIG. 5A is a top view schematic diagram of the drilling of a horizontalwell, shown in FIG. 1 as 110, to form an intersection ornear-intersection with a vertical well, shown in FIG. 1 as 105,according to an illustrative aspect of the invention. Similarly, FIG. 5Bis a cross sectional view taken along the line A to A′ of FIG. 5A anddepicting the drilling of a horizontal well 110 to form an intersectionor near-intersection with a vertical well 105, according to anillustrative aspect of the invention. FIG. 5B is a view along the pathtaken by the horizontal well 110 in progress, and FIG. 5B depicts theformation of the intersection or near-intersection between thehorizontal well 110 and the vertical well 105 at an enlarged section 115of the vertical well 105.

FIG. 6 is a top or plan view schematic diagram of adjustments that maybe made during the drilling of a horizontal well 110 to form anintersection or near-intersection with a target vertical well 105. Inother words, FIG. 6 illustrates how a directional driller utilizes thecapability for determining range and/or direction in order to guide thedrilling of the horizontal well 110. With reference to FIG. 6, from astarting location 600 of the drilling assembly 120, the currentdirection 405 of the drilling assembly 120 is shown. The currentdirection 405 can be the direction from the drilling assembly 120 to atarget position 610 of the vertical well 105 that is based on a surveytaken at the surface or on some other estimated location of the verticalwell 105. Also shown in FIG. 6 is the target direction 420 towards thetarget vertical well 105. A directional driller that is guiding thedrilling assembly 120 can follow or approximately follow the currentdirection 405 until the drilling assembly 120 reaches a distance 605from the vertical well 105 at which the one or more sensors 135 maydetect the alternating current magnetic field.

The distance 605 may be referred to as the detection range of the one ormore sensors 135. The detection range 605 of the one or more sensors isdependent upon the type or types of sensors that are utilized. Theapproximate detection range 605 is typically a known value that isassociated with one or more of the sensors; however, the detection range605 may be determined and/or verified by any suitable method ortechnique. Additionally, it will be appreciated that the detection range605 may vary according to a variety of factors such as, for example, thedensity of the earth and/or other materials that are situated betweenthe drilling assembly 120 and the target vertical well 105.

Once the drilling assembly 120 reaches or is within the detection range605, the directional driller can adjust the path of the drillingassembly based on the measurements taken by the one or more sensors 135.The directional driller can adjust the path in order to form anintersection or near-intersection between the horizontal well 110 andthe vertical well 105. An enlarged section 115 of the vertical well 105may be situated at the intersection or near-intersection. As shown inFIG. 6, the drilling of the horizontal well 110 can be guided along anadjusted path 615 based at least in part on the measurements taken bythe one or more sensors 135 in order to form the intersection ornear-intersection.

According to an aspect of the invention, the measurements taken by theone or more sensors 135 are processed by one or more control units,similar to 305 in FIG. 7, in order to provide the directional drillerwith appropriate information associated with an accurate direction andan accurate distance or range from the drilling assembly 120 to thetarget vertical well 105. The processed measurements and/or data can beprovided to the directional driller in an appropriate format thatfacilitates the guiding of the drilling assembly 120 by the directionaldriller.

FIG. 7 depicts a block diagram of an exemplary control unit 305 utilizedin accordance with the invention in order to determine the direction anddistance from the drilling assembly, such as 120, to a target verticalwell, such as 105. The control unit 305 can be a control unit associatedwith the drilling assembly 120. A similar control unit can be associatedwith the directional driller at the surface. It will be appreciated thataspects of the invention may utilize any number of control units. Forexample, certain aspects may utilize a single control unit.

The control unit 305 of FIG. 7 includes a memory 705 and a processor710. The memory 705 stores programmed logic 715 (e.g., software) inaccordance with the invention. One example of software or acomputer-readable medium is program code or a set of instructionsoperable to receive and process measurements data in order to determinea distance and/or direction from the drilling assembly, such as 120, toa target vertical well, such as 105. The memory 705 also includes data720 utilized in the operation of the aspect of the invention, and alsoincludes an operating system 725. The data 720 can include measurementsdata taken by the one or more sensors, such as 135, in the drillingassembly 120. The processor 710 utilizes the operating system 725 toexecute the programmed logic 715, and in doing so, may also utilize thedata 720. A data bus 730 provides communication between the memory 705and the processor 710. Users can interface with the control unit 305 viaone or more user interface device(s) 735 such as a keyboard, mouse,control panel, or any other devices capable of communicating digitaldata to or from the control unit 305.

The control unit 305 can communicate with external devices such as, forexample, the one or more sensors 135, via one or more appropriateinterface devices 740. The one or more interface devices 740 can alsofacilitate the output of data by the control unit 305 to one or moresuitable output devices such as, for example, a display, and/or to oneor more other system components or external devices. It will beappreciated that communication with external devices may be facilitatedwith any suitable data communication technique such as, for example,communication via a direct connection, communication via a wired networkconnection, communication via a wireless network connection and/orcommunication via a cellular network connection. Further the controlunit 305 and the programmed logic 715 implemented thereby may comprisesoftware, hardware, firmware or any combination thereof.

FIG. 8 is an exemplary flowchart of example general operations taken bya control unit, such as 305, in accordance with an illustrative aspectof the invention. It will be appreciated that some or all operationsdescribed herein can be achieved by a single control unit or by acombination of control units utilized in accordance with the invention.Once the control unit, such as 305, commences operations, the controlunit 305 goes to block 805 and receive measurements from the one or moresensors, such as 135, in the drilling assembly, such as 120. Thereceived measurements can be associated with the alternating currentmagnetic field that is generated by the wireline, such as 205, in thevertical well, such as 105. For example, the received measurements canbe associated with a strength of the alternating current magnetic field.

At block 810, the control unit 305 determines a distance from thedrilling assembly 120 to the target vertical well 105. At block 815, thecontrol unit 305 determines a direction from the drilling assembly 120to the target vertical well 105. The path of the drilling assembly canbe adjusted based at least in part on the determined direction anddistance or range in order to facilitate an intersection ornear-intersection between the horizontal well 110 and the targetvertical well 105.

At block 820, the distance and/or direction determinations canoptionally be adjusted. The adjustments can place the determinations ina more appropriate form for subsequent processing or actions that may betaken based upon the determinations. For example, if the directionaldriller is an individual and the distance determinations are made usingmetric units (e.g., meters), then the distance determinations can beadjusted to standard units (e.g., feet) before they are communicatedand/or displayed to the directional driller. As another example, if thedirectional driller is an individual, then the direction determinationcan be adjusted or corrected to magnetic north prior to beingcommunicated and/or displayed to the directional driller.

For example, the direction determination includes an approximate angle θ425 representing the difference between the current direction 405 of thedrilling assembly 120 and the target direction 420 to the vertical well105. The angle θ can be corrected to magnetic north in order to providethe directional driller with a desired heading for the drilling of thehorizontal well 110 in order to achieve an intersection ornear-intersection with the target vertical well 105. It will beunderstood that any adjustments made to the distance and/or directiondeterminations are optional and may not be necessary. For example, ifthe path of the drilling assembly 120 is automatically controlled by asuitable device or system such as, for example, a computerized guidancesystem, then adjustments to the distance and/or direction determinationsmay not be necessary.

At block 825, the direction and/or the distance determinations canoptionally be communicated to the directional driller. The communicationto the directional driller can include a communication to a control unitassociated with the directional driller from another control unit suchas, for example, a control unit associated with the drilling assembly120. The communication can also include a communication to anappropriate output device associated with the directional driller suchas, for example, a display associated with the directional driller.

The operations described in FIG. 8 can be performed continuously orperiodically as the horizontal well 110 is drilled. With reference toFIG. 8, the operations of the control unit 305 continue at block 805following the determination of a direction and a distance to a targetvertical well 105, and the optional adjustment and communication ofthese determinations. At block 805, the control unit 305 receives newmeasurements from the one or more sensors 135, and the control unit 305utilizes these new measurements to determine a new or updated directionand/or distance to a target vertical well 105. As shown in FIG. 8, theoperations of the control unit 305 are performed continuously; however,it will be appreciated that the operations of the control unit 305 canbe performed periodically.

For example, the control unit 305 can receive and process measurementsat predetermined time intervals such as, for example, every ten seconds.Many different predetermined time intervals may be utilized inaccordance with aspects of the invention, as will be understood by thoseof skill in the art. Additionally, it will be understood that thecontrol unit 305 can direct the storage of any number of receivedmeasurements, determined values, and/or adjusted values. For example,the control unit 305 can store a measurement or a value in the memory705 of the control unit 305. As another example, the control unit 305can direct an associated memory device to store a measurement or avalue.

It will be appreciated that the operations described above withreference to FIG. 8 do not necessarily have to be performed in the orderset forth in FIG. 8, but instead can be performed in any suitable order.Additionally, it will be understood that, in certain aspects of theinvention, the control unit 305 can perform more or less than all of theoperations set forth in FIG. 8. It will also be appreciated that theoperations set forth in FIG. 8 can be performed by any appropriatecontrol unit or combination of control units such as, for example, acontrol unit associated with the drilling assembly and/or a control unitassociated with the directional driller.

According to an aspect of the invention, a distance or range between thedrilling assembly, such as 120 shown in FIG. 1, and the target verticalwell, such as 105 shown in FIG. 1, is determined. The distance or rangeis determined utilizing one or more measurements that are taken by theone or more sensors, such as 135 shown in FIG. 1, of the drillingassembly, such as 120 shown in FIG. 1. In some aspects of the invention,the one or more sensors 135 include three sensors that measure theintensity of an alternating current magnetic field that is generated bya wireline 205 in the target vertical well 105; however, it will beunderstood that the drilling assembly 120 may include or be associatedwith any number of sensors that are configured to measure the intensityof the alternating current magnetic field.

The three sensors respectively measure the intensity of the alternatingcurrent magnetic field in three directions or dimensions. For example, afirst sensor measures the intensity of the alternating current magneticfield in a direction that roughly corresponds to the current path of thedrilling assembly 120, which can be referred to as the Z direction. Asdiscussed earlier with reference to FIG. 4, the Z direction forms anapproximate right angle with a line 410 that is approximately tangentialto the arc of the alternating current magnetic field 415 that is emittedfrom the wireline 205. The second and third sensors measure theintensity of the alternating current magnetic field in respectivedirections that are perpendicular to the current path of the drillingassembly 120, which can be referred to respectively as the X directionand the Y direction. The X direction and the Y direction areadditionally perpendicular to one another. The three sensors can takescalar and/or vector measurements of the alternating current magneticfield.

Once the intensity of the alternating current magnetic field has beendetermined by the three sensors, a single intensity measurement or valueof the intensity is determined or calculated based on a combination ofthe thee intensity measurements, as will be understood by those of skillin the art. It will be appreciated that other suitable determinations ofthe intensity of the alternating current magnetic field can be utilizedin accordance with aspects of the invention.

The distance or range is then determined or calculated based on one ormore intensity measurements. Many different methods can be utilized inorder to determine the distance or range, as will be appreciated bythose of skill in the art. A few methods are discussed herein by way ofexample only. A first exemplary method assumes a relatively simple modelof the alternating current magnetic field. The first exemplary methodassumes a model in which there is no loss or very little loss of thecurrent present on the wireline 205 into the surrounding vertical well105 (i.e., the current producing the field is precisely known), and forwhich the grounding electrode 210 has a low termination resistancevalue, which creates a resistive path to ground with little or nocurrent loss. In other words, the first exemplary model assumes arelatively ideal or ideal, straight current-carrying wire.

In the first exemplary method, the relationship between the value of thealternating current carried by the wireline 205, the generated fieldintensity, and the distance or range is given by the following equation:

$\begin{matrix}{H = \frac{\mu_{0} \cdot I}{2\; {\pi \cdot r^{n}}}} & (1)\end{matrix}$

where “H” represents the generated field intensity, “I” represents thevalue of the alternating current carried by the wireline 205, “r”represents the distance or range, “n” represents the falloff rate of themagnetic field, and “μ₀” is a constant of 4π*10⁻⁷ T·m/A, or thepermeability of free space. Equation (1) represents a special case ofthe Biot-Savart Law for the magnetic field intensity due to a current ina thin, infinitely long straight conductor. The value of “n” can beassumed to be equal to 1; however, it will be appreciated that othervalues for “n” may be utilized. For example, the value of “n” can changeas the one or more sensors 135 approach the wireline 205 and the diffusecurrents become more concentrated as the distance or range decreases.Simplifying equation (1) yields:

$\begin{matrix}{{H = \frac{2 \cdot I \cdot 10^{- 7}}{r^{n}}}{or}{H = \frac{K \cdot I}{r^{n}}}} & (2)\end{matrix}$

where “K” is a constant of 2×10⁻⁷. Solving for distance or range yields:

$\begin{matrix}{{r^{n} = \frac{K \cdot I}{H}}{or}{r = \left( \frac{K \cdot I}{H} \right)^{- n}}} & (3)\end{matrix}$

This equation can be used to calculate range, and the current and thefield intensity should be known (or closely estimated).

If a low loss wire model is assumed, then equation (3) can be utilizedto determine or calculate the distance or range between the drillingassembly 120 and a target vertical well 105 if the current in thewireline 205 is known and the intensity of the alternating currentmagnetic field is known, accurately measured, and/or closelyapproximated. It will be appreciated that a value for the current can beassumed for the calculation based upon a current that is supplied to thewireline 205 by the alternating current generation source 145.Alternatively, the current can be measured in the wireline 205 by anappropriate current measuring device such as, for example, an ammeter ora current sensing transformer, and the current measurement can becommunicated to a control unit such as, for example, the control unit305 associated with the drilling assembly 120.

The second exemplary method for determining a distance between adrilling assembly 120 and a target vertical well 105 makes no or limitedassumptions about the loss of current from the wireline 205 to thesurrounding vertical well 105 and/or earth. Additionally, the secondexemplary method does not assume that the amplitude of the alternatingcurrent in the wireline 205 is a known amplitude; however, the secondexemplary method can assume that the alternating current in the wireline205 has a constant or approximately constant amplitude. For certainapplications, such as for example, the CBM application 100 depicted inFIG. 1, the wireline 205 may not have a current with an exactly knownamplitude flowing through it. For example, as the wireline 205approaches the grounding electrode 210, the amplitude of the current maynot be exactly known at or near the terminal end of the wireline 205connected to the grounding electrode 210. It will be appreciated that atleast a portion of the current may be lost to the surrounding verticalwell 105 and/or that at least a portion of the current may be diffusedinto the earth.

FIG. 9 is a graphical representation of one example of falloff rates fortwo different alternating current magnetic fields having differentintensities. The falloff rates depicted in FIG. 9 represent the falloffrates of magnetic field intensity over distance as the magnetic fieldspropagate away from a source such as, for example, the wireline 205.With reference to FIG. 9, the field intensity versus distance isillustrated for the two magnetic fields 905, 910. The first magneticfield 905 represents a magnetic field that is generated by a loweramplitude of current in the wireline 205 relative to the current thatgenerated the second magnetic field 910.

At a distance “r₁” from the source (e.g., the wireline 205), theintensity of the first magnetic field 905 is “H_(a1)” and the intensityof the second magnetic field 910 is “H_(b1)”. The slope or gradient ofthe first magnetic field 905 at “r₁” is Δ H_(a1)/Δ r₁, and the slope orgradient of the second magnetic field 910 at “r₁” is Δ H_(b1)/Δ r₁. Thefalloff rate is expressed as an exponential “n” in the denominator. Thefalloff rate of intensity over distance for each field is approximatelythe same and may be given by:

$\begin{matrix}{{FalloffOverDist} = \frac{1}{r^{n}}} & (4)\end{matrix}$

Thus, the distance or range “r1” is independent of the strength of thesource and of the attenuation effect.

According to an aspect of the invention, the second exemplary method fordetermining a distance or range can make use of one or more gradientcalculations. For example, the second exemplary method can make use ofthe measurements of the intensity of the alternating current magneticfield to calculate one or more gradients. A method that utilizes one ormore gradients to determine a distance or range can also be referred toas gradient ranging. Differentiating equation (2) above provides a rateof change of the alternating current magnetic field intensity as thedistance changes given a value of the current in the wireline 205.Differentiating equation (2) leads to the derivative of the filedintensity with respect to distance, which can be referred to as thefield gradient. The field gradient is given by:

$\begin{matrix}{\frac{H}{r} = \frac{{- n} \cdot K \cdot I}{r^{n + 1}}} & (5)\end{matrix}$

where “K” is a constant of 2×10⁻⁷. The derivative can be utilized withthe intensity value at a location to determine or calculate the distanceor range to the target vertical well 105. Taking the ratio of theintensity to the gradient results in an expression for the distance orrange “r” and the falloff rate “n”, given as:

$\begin{matrix}{\frac{H}{\left( \frac{H}{r} \right)} = {{\frac{\left( {K \cdot I} \right)}{- r^{n}} \cdot \frac{\left( r^{n + 1} \right)}{\left( {{- n} \cdot K \cdot I} \right)}} = \frac{- r}{n}}} & (6)\end{matrix}$

From equation (6), “r” is determined as:

$\begin{matrix}{{- r} = {n \cdot \left\lbrack \frac{H}{\left( \frac{H}{r} \right)} \right\rbrack}} & (7)\end{matrix}$

In equation (7), the constant “K” and the current “I” are no longerused. Further, a close approximation for dH/dr can be given by ΔH/Δr.Accordingly, the rate of change of the measured intensity over a changein distance can be utilized in a determination of a distance between thedrilling assembly 120 and a target vertical well 105. In a situation inwhich a single set of sensors 135 is included in the drilling assembly120, the gradient can be measured and/or approximated by utilizing thesensors 135 in at least two positions. At each position, the intensityof the alternating current magnetic field is measured, and adetermination of the distance or range to the target vertical well 105is made based at least in part on the intensity measurements at eachposition. Each measured intensity can be the total intensity of thealternating current magnetic field that is measured at each position.Additionally, at each position, a depth of the drilling assembly 120from the surface can be inputted and/or measured. The value of “n” canalso be inputted and/or entered. It will be appreciated that apredetermined value for “n” can be inputted such as, for example, adefault value of one.

According to an aspect of the invention, the measurements can be made inmany different modes of operation for the drilling assembly 120 such as,for example, a drilling mode, pushing on the bit, or in a pulling-backor off-bottom mode. One or more sets of data can be taken to improve theaccuracy of the distance or range determination. One or more sets ofdata can be taken at one or more locations or positions of the drillingassembly 120. It will be appreciated that a plurality of determinationsbased on a plurality of data sets can be averaged together to improvethe accuracy of the determinations.

FIG. 10A is an exemplary flowchart depicting exemplary operations takenby the control unit, such as 305 of FIG. 7, to determine a distance orrange to a target vertical well 105, in accordance with an illustrativeaspect of the invention. The operations depicted in FIG. 10A utilize adrilling assembly 120 that is operating in a pulling-back or off-bottommode. At block 1005, the drilling assembly 120 is located or stopped ata first position “P_(a)”. Point “P_(a)” is a point that is situated at adesired depth. At point “P_(a)”, the drill bit 130 is pulled off thebottom by an appropriate distance such as, for example, by one or twofoot. At point “P_(a)”, the distance or range to the target verticalwell 105 is given as “r_(a)”. At block 1010, data is acquired from theone or more sensors 135 and the total magnetic field intensity “H_(a)”is determined by the control unit 305. The data and/or the fieldintensity “H_(a)” is stored for later use at block 1015.

At block 1020, the drilling assembly 120 is moved to a differentposition “P_(b)” in the horizontal well 110. For example, the drillingassembly 120 is moved forward in the horizontal well 110 by apredetermined distance “Δr₁”. The predetermined distance “Δr₁” can beany appropriate distance such as, for example, a distance that is muchsmaller than the distance or range from the drilling assembly 120 to thetarget vertical well 105. At position “P_(b)”, the distance or range tothe target vertical well 105 is given as “r_(b)”. At block 1025, data isacquired from the one or more sensors 135 and the total magnetic fieldintensity “H_(b)” is determined by the control unit 305. The data and/orthe field intensity “H_(b)” is stored for later use at block 1030.

At block 1035, the drilling assembly 120 is moved to a differentposition “P_(c)” in the horizontal well 110. For example, the drillingassembly 120 is moved forward in the horizontal well 110 by apredetermined distance “Δr₂”. The predetermined distance “Δr₂” can beany appropriate distance such as, for example, a distance that is muchsmaller than the distance or range from the drilling assembly 120 to thetarget vertical well 105. The predetermined distance “Δr₂” can beapproximately equal to the distance “Δr₁”; however, it will beunderstood that “Δr₂” may be a different distance than “Δr₁”. Atposition “P_(c)”, the distance or range to the target vertical well 105is given as “r_(c)”. At block 1040, data is acquired from the one ormore sensors 135 and the total magnetic field intensity “H_(c)” isdetermined by the control unit 305. The data and/or the field intensity“H_(c)” is stored for later use at block 1045.

At block 1045, several data values are stored by the control unit 305for later use. For example, values for “P_(a)”, “P_(b)”, and “P_(c)” arestored. The corresponding depths at “P_(a)”, “P_(b)”, and “P_(c)” canrespectively be “r₁”, “r₂”, and “r₃”. Additionally, the values can bereferenced to the same point such as, for example, to the drill bit 130of the drilling assembly 120. Values for “n” and “K” can also be storedby the control unit 305. The value for “n” can be set to a default valuesuch as, for example, to 1.0. The value of “K” can be 2×10⁻⁷. It will beappreciated that other values can be stored by the control unit 305.These other values may include values utilized in calculations otherthan those for gradient ranging. These other values may include forexample, a value of the current “I” that is present in the wireline 205.

At block 1050, values for the average intensity between two positionsand the difference in intensity for two positions is calculated by thecontrol unit 305 according to the following equations:

$\begin{matrix}{{H_{{avg}\; 1} = {{\frac{\left( {H_{a} + H_{b}} \right)}{2}\mspace{14mu} \Delta \; H_{1}} = {H_{b} - H_{a}}}}{H_{{avg}\; 2} = {{\frac{\left( {H_{b} + H_{C}} \right)}{2}\mspace{14mu} \Delta \; H_{2}} = {H_{c} - H_{b}}}}} & (8)\end{matrix}$

At block 1055, one or more gradient ranging calculations are determinedby the control unit 305 according to the following equations:

$\begin{matrix}{{{Range}_{{grad}\; 1} = {{n \cdot \frac{H_{{avg}\; 1}}{\left( \frac{\Delta \; H_{1}}{\Delta \; r_{1}} \right)}} - \frac{\Delta \; r_{1}}{2}}}{{Range}_{{grad}\; 2} = {{n \cdot \frac{H_{{avg}\; 2}}{\left( \frac{\Delta \; H_{2}}{\Delta \; r_{2}} \right)}} - \frac{\Delta \; r_{2}}{2}}}} & (9)\end{matrix}$

The determined gradient range values are referenced to position “P_(a)”.Additionally, the values can be corrected to the depth of the drillingassembly 120.

At block 1060, the two gradient range values are averaged by the controlunit 305 according to the following equation:

$\begin{matrix}{{Range}_{avg} = \frac{\left( {{Range}_{{grad}\; 1} + {Range}_{{grad}\; 2}} \right)}{2}} & (10)\end{matrix}$

The determined value for Range_(avg) represents the distance or rangefrom the drilling assembly 120 to the target vertical well 105.

It will be appreciated that the operations described above withreference to FIG. 10A do not necessarily have to be performed in theorder set forth in FIG. 10A, but instead may be performed in anysuitable order. Additionally, it will be understood that, in certainaspects of the invention, the control unit 305 can perform more or lessthan all of the operations set forth in FIG. 10A. It will also beappreciated that the operations set forth in FIG. 10A can be performedby any appropriate control unit or combination of control units such as,for example, a control unit associated with the drilling assembly and/ora control unit associated with the directional driller.

FIG. 10B is a graphical representation of exemplary distances and valuesthat can be measured and/or determined by utilizing the exemplaryoperations depicted in FIG. 10A.

According to another aspect of the invention, a direction from thedrilling assembly 120 to the target vertical well 105 is determined. Theone or more sensors 135 are utilized to measure the components of thealternating current magnetic field. The measured components of thealternating current magnetic field are utilized to determine orcalculate a relative direction angle “θ” 425 between the current path ofthe drilling assembly 120 and the actual direction to the targetvertical well 105.

According to an aspect of the invention, the components of thealternating current magnetic field are measured in three directions. Asensor “k” measures the component of the field in the Z direction, asensor “i” measures the component of the field in the X direction, and asensor “j” measures the component of the field in the Y direction. Thevalues measured by the “i” and “j” sensors can be resolved into a“radial” value. The radial value is a vector that lies in a plane thatis situated at a right angle to the direction of the borehole of thehorizontal well 110. The radial value is given by:

radial=√{square root over ((i ² +j ²))}   (11)

The value measured by the “k” sensor can be referred to as an “axial”value. The value measured by the “k” sensor is a value that is alignedwith the borehole or the current path of the drilling assembly 120. Thedirection to the target “θ” 425 is determined by the following equation:

$\begin{matrix}{\theta = {\tan^{- 1}\left( \frac{axial}{radial} \right)}} & (12)\end{matrix}$

In the event that the current path of the drilling assembly 120coincides or very nearly coincides with the direction “θ” 425 to thetarget vertical well 105, the tangent field will form a right angle withthe axial sensor “k” and the output of the sensor “k” will be zero.Additionally, the phase of the axial sensors can change 180 degrees asthe axial null orientation is passed in rotating the current path of thedrilling assembly 120 from left side of “θ” 425 to the right side of “θ”425, or vice versa.

The determined value of “θ” 425 can be a value that is relative to thecurrent path of the drilling assembly 120. According to an aspect of theinvention, the value of “θ” 425 can be adjusted. For example, the valueof “θ” 425 can be corrected to magnetic north so that a directionaldriller may be presented with familiar terminology while steering thedrilling of the horizontal well 110.

FIG. 11 is an exemplary flowchart depicting exemplary operations takenby a control unit, such as 305 in FIG. 7, at block 815 of FIG. 8 todetermine a direction to a target vertical well 105, in accordance withan illustrative aspect of the invention. At block 1105, the componentsof the alternating current magnetic field are measured by one or moresensors 135 and communicated to the control unit 305. At block 1110, themeasurements are stored by the control unit 305. At block 1115, thecontrol unit 305 determines a radial value according to equation (11)above. At block 1120, the control unit 305 determines an axial value inthe direction of the current path of the drilling assembly 120. At block1125, the control unit 305 determines the direction to the targetvertical well 105, or “θ” 425, based at least in part on the radialvalue and the axial value.

It will be appreciated that the operations described above withreference to FIG. 11 do not necessarily have to be performed in theorder set forth in FIG. 11, but instead can be performed in any suitableorder. Additionally, it will be understood that, in certain aspects ofthe invention, the control unit 305 can perform more or less than all ofthe operations set forth in FIG. 11. It will also be appreciated thatthe operations set forth in FIG. 11 can be performed by any appropriatecontrol unit or combination of control units such as, for example, acontrol unit associated with the drilling assembly and/or a control unitassociated with the directional driller.

Many modifications and other aspects of the invention set forth hereinwill come to mind to one skilled in the art to which the inventionpertains having the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificaspects disclosed and that modifications and other aspects are intendedto be included within the scope of the appended claims. Althoughspecific terms are employed herein, they are used in a generic anddescriptive sense only and not for purposes of limitation.

1. A method for guiding the drilling of a horizontal well, comprising:providing a current in at least one conductor positioned in a targetvertical well; measuring, at a drilling assembly that is drilling thehorizontal well, a magnetic field generated by the current; determining,based at least in part on the measured magnetic field, a direction fromthe drilling assembly towards the target vertical well; and determining,based at least in part on the measured magnetic field, a distance fromthe drilling assembly to the target vertical well, wherein determiningthe distance comprises determining at least one gradient.
 2. The methodof claim 1, wherein providing a current comprises providing a lowfrequency alternating current by a current generation source.
 3. Themethod of claim 2, further comprising: terminating the at least oneconductor to earth ground at a distal end of the at least one conductorthat is opposite to an end that is terminated to the current generationsource.
 4. The method claim 1, further comprising: outputting at leastone of the determined direction and the determined distance to adirectional driller controlling the path of the drilling assembly. 5.The method of claim 4, wherein outputting at least one of the determineddirection and the determined distance comprises displaying thedetermined direction, and further comprising: correcting the determineddirection to true north prior to displaying the determined direction. 6.The method of claim 1, further comprising: adjusting the path of thedrilling assembly based at least in part on one or more of thedetermined distance and the determined direction.
 7. The method of claim1, wherein determining the at least one gradient comprises: determininga rate of change of the magnetic field as the distance from the drillingassembly to the target vertical well changes.
 8. The method of claim 7,wherein determining the rate of change of the magnetic field as thedistance from the drilling assembly to the target vertical well changescomprises: measuring the intensity of the magnetic field at a firstposition of the drilling assembly in the horizontal well and at a secondposition of the drilling assembly in the horizontal well; determining adifference in intensity between the measured intensity at the firstposition and the measured intensity at the second position; determininga distance between the first position and the second position; anddetermining the rate of change of the intensity of the magnetic field asthe distance from the drilling assembly to the target vertical wellchanges based at least in part on the determined difference in intensityand the determined distance between the first position and the secondposition.
 9. The method of claim 1, wherein determining the at least onegradient comprises: determining a plurality of gradient calculations;and averaging the plurality of determined gradient calculations.
 10. Themethod of claim 1, wherein determining the direction from the drillingassembly to the target vertical well comprises: determining a pluralityof components of the measured magnetic field along respective axes;determining a radial value of the direction based at least in part on atleast one of the plurality of components; determining an axial value ofthe direction based at least in part on at least one of the plurality ofcomponents; and determining the direction based at least in part on thedetermined radial value and the determined axial value.
 11. A system forguiding the drilling of a horizontal well, comprising: at least oneconductor positioned in a target vertical well and operable to carry acurrent signal; one or more sensors associated with a drilling assemblythat is drilling the horizontal well, wherein the one or more sensorsare operable to measure the intensity of a magnetic field generated bythe current signal; and a processor operable (i) to receive theintensity measurements from the one or more sensors, (ii) to determine adirection from the drilling assembly towards the target vertical wellbased at least in part on the received intensity measurements, and (iii)to determine a distance from the drilling assembly to the targetvertical well based at least in part on the received intensitymeasurements, wherein at least one gradient calculation is utilized todetermine the distance.
 12. The system of claim 11, wherein the currentsignal comprises a low frequency alternating current signal generated bya current generation source.
 13. The system of claim 12, wherein the atleast one conductor is terminated to earth ground at a distal end of theat least one conductor that is opposite to an end that is terminated tothe current generation source.
 14. The system claim 11, furthercomprising at least one output device operable to display at least oneof the determined direction and the determined distance to a directionaldriller controlling the path of the drilling assembly.
 15. The system ofclaim 14, wherein: the at least one output device is operable to displaythe determined direction; and the control unit is further operable tocorrect the determined direction to true north prior to the display ofthe determined direction.
 16. The method of claim 11, wherein the pathof the drilling assembly is adjusted based at least in part on one ormore of the determined distance and the determined direction.
 17. Thesystem of claim 11, wherein the processor determines the at least onegradient calculation by determining a rate of change of the intensity ofthe magnetic field as the distance from the drilling assembly to thetarget vertical well changes.
 18. The system of claim 17, wherein theone or more sensors are operable to measure the intensity of themagnetic field a first position of the drilling assembly in thehorizontal well and at a second position of the drilling assembly in thehorizontal well; and the processor determines the rate of change of theintensity of the magnetic field as the distance from the drillingassembly to the target vertical well changes by: determining adifference in intensity between the measured intensity at the firstposition and the measured intensity at the second position; determininga distance between the first position and the second position; anddetermining the rate of change of the intensity of the magnetic field asthe distance from the drilling assembly to the target vertical wellchanges based at least in part on the determined difference in intensityand the determined distance between the first position and the secondposition.
 19. The system of claim 11, wherein the at least one gradientcalculation comprises a plurality of gradient calculations that areaveraged together.
 20. The system of claim 11, wherein the processordetermines the direction from the drilling assembly towards the targetvertical well by: identifying a plurality of components of the measuredintensity along respective axes; determining a radial value of thedirection based at least in part on at least one of the identifiedplurality of components; determining an axial value of the directionbased at least in part on at least one of the identified plurality ofcomponents; and determining the direction based at least in part on thedetermined radial value and the determined axial value.